Field of the Invention
This invention relates to the role of phytochrome genes in the regulation of flowering, fiber initiation and elongation, and other characteristics affected by altered photomorphogenesis in Gossypium plants; PHYA1 gene silencing constructs comprising polynucleotides encoding phytochrome A1 proteins, transgenic cotton plants comprising the PHYA1 RNAi polynucleotides, and a method of using RNA interference of the phytochrome PHYA1 gene to generate novel transgenic plants exhibiting improved cotton fiber quality, early-flowering and early boll maturity, enhanced root elongation, and increased seed cotton production due to both the suppression of PHYA1 and the several fold increases in the expression of other phytochrome genes.
Description of the Relevant Art
Light is one of the most important environmental factors controlling plant development and physiology. It affects virtually all aspects of plant growth, from seed germination to vegetative morphology, floral initiation, control of circadian rhythms, gene regulation and expression, gravitropism and phototropism (Fankhauser and Chory. 1997. Ann. Rev. Cell Dev. Biol. 13:203-229; Furuya and Kim. 2000. Trends in Plant Sci. 3:87-88; Tepperman et al. 2001. Proc. Natl. Acad. Sci. USA 98(16):9437-9442). Plants respond to light through several photoreceptor systems. The phytochrome photoreceptor gene family is best characterized in the model plant Arabidopsis, which has five phytochrome genes PHYA, PHYB, PHYC, PHYD, and PHYE (Sharrock and Quail. 1989. Genes and Dev. 3:1745-1757; Clack et al. 1994. Plant Mol. Biol. 25:413-427; Cowl et al. 1994. Plant Physiol. 106:813-814). The phytochromes interact with cryptochromes, the circadian clock, phytohormones, and other signals to regulate floral initiation (Devlin et al. 1998. Plant Cell 10:1479-1487; Devlin et al. 1999. Plant Physiol. 119:909-915; Koornneef et al. 1997. Plant Cell & Environ. 20:779-784; Koornneef et al. 1998. Ann. Rev. Plant Physiol. Plant Mol. Biol. 49:345-370). In Arabidopsis, PHYA promotes plant flowering. A mutation in this gene causes a late flowering phenotype in Arabidopsis (Neff and Chory. 1998. Plant Physiol. 118:27-35). In contrast, PHYB is an inhibitor of flowering induction (Koornneef et al. 1998, supra; Reed et al. 2000. Plant Physiol. 122:1149-1160). Mutations in PHYB cause early flowering in both short (SD) and long (LD) day conditions in Arabidopsis (Bagnall et al. 1995. Plant Physiol. 108: 1495-1503), pea (Mockler et al. 1999. Dev. 106:2073-2082) and sorghum (Childs et al. 1997. Plant Physiol. 97:714-719). Plants overexpressing PHYA, being hyposensitive to photoperiod, exhibit light-dependent dwarfism, darker green leaves, reduced apical dominance and an early flowering phenotype in both SD and LD conditions (Bagnall et al., supra). PHYB/D/E overexpression correlates with shortening of hypocotyl length (Clough et al. 1995. Plant Physiol. 109:1039-1045; Devlin et al. 1999, supra; Devlin et al. 1998, supra; Lin, C. 2000. Plant Physiol. 1239:39-50) and an early flowering phenotype, as is observed, for example, in phyb mutants, suggesting more complex action mechanisms for PHYB (Bagnall et al., supra; Lin, supra). PHYC also contributes to photoperiodic flowering and natural phenotypic variation in flowering time in Arabidopsis (Franklin et al. 2003. Plant Cell 15:1981-1989; Monte et al. 2003. Plant Cell 15:1962-1980; Balasubramanian et al. 2006. Nat. Genet. 38:711-715). Additionally, phytochrome genes regulate vegetative plant growth parameters such as height, leaf and rosette production (Bagnall et al., supra).
In cultivated cottons, the phytochrome gene family has additional importance because there is evidence that the far red/red (FR/R) photon ratio influences length and diameter of developing fiber. For example, cotton fibers that were exposed to a high far red/red photon ratio were longer than those exposed to elevated photosynthetic light (Kasperbauer, M. J. 1994. Physiol. Plantarum 91:317-321; Kasperbauer, M. J. 2000. Crop Sci. 40:1673-1678). Genetic improvement of fiber yield and fiber quality, i.e., fiber length and fiber strength, is the primary objective of cotton breeding programs worldwide (Perkins et al. 1984. In: Cotton Agron. Monogr. Kohel and Lewis, Eds., ASA, CSSA, and SSSA, Madison, Wis., pp. 437-509). Fiber quality has become a major issue in recent years because of the technological changes in the textile industry (Perkins et al., supra; El-Mogahzy and Chewning. 2001. In: Cotton Fiber to Yarn Manufacturing Technology. Cotton Incorporated, Cary, N.C.). Pima (Gossypium barbadense) cotton fibers are fine, genetically stronger, and more uniform than the widely grown, early maturing and high yielding Upland (Gossypium hirsutum) cottons (El-Mogahzy and Chewning, supra). Finding an easy way to improve fiber properties of Upland cultivars, while maintaining yield and early maturity, is a fundamental problem to be solved in conventional cotton breeding worldwide.
Thus, there is a need for the development of improved cultivated cotton plants which produce high yields of quality cotton fibers which exhibit improved fiber length and fiber strength.